Manufacture of chromium alloy steel



Patented Sept. 28, 1926.

. UNITED" s A BYnAIn D. sAKL rwALLA, or-cRArroN, rmrnsrhvmm.

muracrunn or cnnomum Armor s'rnnn.

No Drawing.

erto, the chromium content was generally introduced in the form of a ferro-alloy. :As the regular commercial grades of. ferrochromium contain an appreciable percentage of carbon, this practice introduced anundue amount of carbon in the steel. This is par ticularly objectionable in the so-called stainless steels or irons which have a high chromiumcontent, usually in the neighbor;

hood of 9% to 15%, because the presence of carbon in such'steels has a greater physical efiect than the-same amount of carbon'in ordinary steel; for example 1/10of 1% of carbon in the high chromium stainless steel hasabout the same physical effects as 4/10.%

- of carbon in the ordinary steel. A stainless iron should contain less than 1/10 of 1% of carbon.

It is not commercially practicable to decarburize the steel after the addition of thecarbon-containing ferro-chromium, because.

the processes of oxidizing the carbon are accompanied by an oxidation and consequent waste of the valuable chromium content.

On the other hand,if'special grades of low ferro-chromium are used, their cost is so high as to prohibitively increase hecOfSt of the steel produced. The defects 0 disadvantagesin adding the commercial: grades of ferro-chromium or the special grades of low-carbon ferro-chromium are particularly apparent in the manufacture of -the so-called stainless steels or irons, where alarge quantity of ferro-chromiu'm has to be added, since the percentage of chromium must be high, usually between '9%' and 15%' andthe carbon content-should be-very low. V

In accordance with the present invention,

the carbon content ofthesteel is greatly reduced by the addition of the chromium com chromium steel, under the prevailing practice existing hith- Application fled March 11, 1922. Serial No. 543,081.

pound or ore, and the chromium may be obtained from a source very much cheaper than ferro-chromium. The chromium is added preferably in the form of chrome or chromite to a bath of molten steel in any TES PATENTOFF-ICE. 7

ordinary steel-making furnace, and chromi- 1 um 1s reduced from thechrom-ite by means of the carbon, and possibly other reducing agents present in the 'steel. By this means, the chromium is introduced into the steel, and at the same time, the carbon content is eliminated or reduced to the desired minimum. Theprocess may be carried out in the following mannerr Pig 1101101 pig 1ron and scrap are meltedin an ordinary steel-making furnace, such as. an open-hearth furnace. After the metal -is molten, the requisite quantity of chrome. ore, together with lime and fiuorspar, as

fluxes, is'charged on top of the molten bath.

When the "reaction between the chromium oxide of the chromium ore and the carbon 1 in the bath ceases, the metal is tapped.

The lime and fiuorspar added with. 'the chrome ore as fluxes help to refine the v finished steel from impurities, such as sul-- phur and phosphorus.- This refining can also be further controlled by increasing the furnace temperature before tapping. The fluorspar added with t e lime makes a more liquid slag and is adde because the alumina and magnesia in the chromite, although,

forming a basic slag, do 'not make a suf ficien'tly liquid slag.

.If a considerable proportlon of chromium is to, be added the form of chrome ore,

the amount of the reducing-agent in the steel may be increased either by addingv I pig iron or other high carbon irons or steels,

or by introducing another reducing agent, I

{such as silicon or manganese, into the bath. 'The carbon exists in the pig iron or other 11 high carbon irons orsteels principally in the form of iron carbide. I S'ili'con may be added in the form eof ferro-silicon, rmay be-in the form of a chrome silicon alloy. Such a chrome silicon alloy may be cheaply prepared and may contain in the neighborhood of-30% to 35% silicon, 4.0% chromium,

and the remainder iron and impurities. 'The amount of chromite or chrome ore added carbon or the carbon and silicon content to the desired minimum. The slag'will' contain some chromium or chromite which be wasted, butthis is not a serious objection should be in sufiicient quantity to reduce the bon and silicon are not sufliciently reduced,

more cltromite may be added, since the limits of tolerance of the chromium generally vary over a few percent; In this way, a much more, minute andaccurate control of the carbon and silicon limits may be secured than would be the case if they were introduced into. the steel at the same time as the chromite ore.

If desired, the carbon or silicon content may be slightly increased after the oxidizing reaction of the chromite ore, takes place, for example, as by the addition of a small amount of ferro-silicon to deoxidize the steel, and reduce a further amount of the chromium.

The process is intended principally as an open-hearth or electric furnace process, although it may be used with a crucible or any other steelmaking furnace.

The source of chromium and oxygen which causes the reaction to reduce the carbon or carbon and silicon and add,chromiurn is preferably chromite on account of its cheapness, although'other sources of chr0- mium may be used.

I The-silicon if used may be added in the form of ferro-silicon or chrome silicon alloy; and the term silicon as herein employed, is intended to cover various forms of silicon or-silicon-bearing materials or al- The term ,steel as herein employed is intended as a-term of general description and not of limitation, and is intended to include the so-called stainless irons which are, in reality, steels having a very low carbon and fmetalloid content. The term chrome steel is also intended as a term of general description and not of limitation, and is intended to include steels having chromium as the principal alloying metal, although containing other alloying metals.

The tolerance limitation of the alloying; metals and the anetalloids in steels is very-1 much closer than in the so-called fer'ro alloys, such for example, as ferrochromiunr, and it is therefore necessary to carry out: steel making processesunder much more carefully regulated and controlled conditions than processes of making ferro alloys.

In carrying out my process the carbon, which is themajor reducing agentfis present in which the amount of the carbonaceous I reducing agent is predetermined and carefullyregulated.

The carbon reducing agent may be incorporated into the molten metal of the bath by meltingdown a charge containing the requisite amount of carbon, or the carbonaceous reducing agent may be added later to the completed bath in regulated amounts, preferably in the form of pig iron or other high carbon irons or steels which of course have a sufficient specific gravity to sink through the slag layer and carry the car bon into the metal layer. An unreduced chromium compound, such for example, as chromite, may be introduced into the slag layer at any suitable or convenient time, either during the melting down of the charge or after the bath is completely formed. The specific gravity of these unreduced compounds is such that they will naturally gravitate to a'ndbecome incorporated in the slag layer. r

The reducing reaction apparentl takes place across the interface betweenfthe metal layer and the slag layer. The reduced chromium in its .nascent'state is in contact with the steel bath and therefore the best conditions are obtained for the alloying of the reduced chromium and its dissemination throughout the steel.

While the preferred embodiment of the present invention has been specifically described, it is to be understood that the present invention is not limited to the herein in which is incorporated chrome ore and maintaining the .bath in a. molten condition so as to cause a-reaction between the carbon in the -metal layer and the chrome ore in the "slag layer to thereby directly reduce chromium into the steel, substantially as described. i i i '2. Those steps in' the process of making chrome steel, which comprise introducing the major portion, if not all, of the chromium into the steel by forming a molten bath I cause a reaction between the carbide in theof steel having a metal layer and a slag layer, adding a carbide .to the metal layer in an amount predetermined .by the desired chromium content of the 'steel and adding chrome ore'to the slag layer, and maintaining the bath in a molten condition so as to metal layer and the chrome ore in the slag layer to thereby directly introduce the reduced metallic chromium into the steel, sub stantially as described.

3. Those steps in the process of making chrome steel, which comprise forming a molten bath of steel having a metal layer containing carbon as the principal reducing agent and a slag layer in which is. incorporated an unreduced chromium compound,.maintaining the bath in a molten conditiomso as to cause a reaction between the carbon in the metal layer and the unreduced chromium compound in the slag layer to thereby directly reduce the chromium into the steel, and thereafter increasing the temperature of the bath to further refine the.

chrome steel, which comprise introducing the major portion, if not all, of the chromium into the steel by forming a molten bath of steel having a metal layer containing carbon as the major reducing agent in an amount thereby directly reduce into the steel the chromium from its ore simultaneously with the refining of the steel, substantially as described.

5. Those steps in the process of making chrome steel, which comprise introducing the major portion, if not all, of the chromium into the steel by forminga molten bath of steel having a metal layer containing carbon as the major reducing agent and aslag layer in which is incorporated an unreduced chromium compound, maintaining the bath. in a molten condition so as to cause a reaction between the carbon in, the metal layer andthe unreduced chromium compound in theslag layer to therebydirectly reduce chromium into steel,.' and thereafter adding a silicon reducing agent to the steel to reduce more of the chromium, substantially as described. I

In testimony whereof I have hereunto set my hand.-

BYRAMJI D. SAKLATWALLA.- 

